1. Field of the Invention
The present invention pertains to a magnetic sensor and associated circuit arrangement for a pipeline survey pig for detecting girth welds and other magnetic anomalies in fluid transmission pipelines.
2. Background
The development of pipeline survey pigs for accurate determination of the location or change in location of fluid transmission pipelines has become increasingly important with the continuing growth of pipeline networks and the desire to minimize the risk of pipeline failure and the consequent economic and environmental damage. One major improvement in pipeline survey pigs is disclosed and claimed in U.S. Pat. No. 4,747,317, issued May 31, 1988 in the name of Pedro F. Lara and assigned to the assignee of the present invention. The survey pig disclosed in the above identified patent application utilizes an inertial sensing unit to determine the course of a pipeline through which the pig is traversed and/or to determine any change in course of the pipeline from a previous survey. The survey pig described in the above identified application would also benefit from an accurate velocity and longitudinal position determining system, particularly in pipelines which have known position markers which may be sensed by a magnetic circuit.
For example, certain fluid transmission pipelines have well documented locations and spacings of circumferential girth welds which are used to join the sections of pipe together. Accordingly, a detection system which can accurately determine when a girth weld is being passed by the survey pig can utilize the location detection signal to accurately measure survey pig velocity, longitudinal position in the pipe and, as determined in pursuing the present invention, certain characteristics of the pipe wall and whether or not any axial strain is being encountered by the pipe which will result in elongation or compression of the pipe sections.
Certain problems in the art of pipeline survey pigs and associated detection apparatus have been required to be overcome in pursuing the present invention. For example, it has been determined that it is highly desirable to provide a magnetic sensing device which will detect a magnetic anomaly caused by a pipeline girth weld, which device can be located in proximity to the wall surface of the pipe, can provide a well defined electrical signal and which physically occupies a minimum amount of space. One preferred type of pipeline survey pig utilizes a generally cylindrical housing which is supported in the pipeline by a plurality of axially spaced apart generally annular resilient cup-like support members. These cup members are adapted to support the pig body in the pipe while providing for propulsion of the pig through the pipe by the force of fluid acting on one or more of the cup members. Accordingly, it has been considered desirable to provide a magnetic field sensing circuit which may be mounted in proximity to the pipe wall on one or more of the resilient cup members or similar structure and which can provide an electrical signal which may be modified to become a suitable digital input signal to a microprocessor based control and recording system for the survey pig.
Another problem associated with developing a suitable girth weld detection system for pipeline survey pigs relates to providing a system which will accurately determine the presence of a girth weld but which will avoid giving false readings which may result from spiral or longitudinal welds as well as magnetic anomalies caused by pipeline repairs, various devices interposed in the pipeline, such as valves, and other structures in the vicinity of the pipe which might generate a sufficiently intense magnetic field to provide a false signal from the girth weld detection circuit. The problems recognized in the art of magnetic weld detector systems and the advantages sought for such a system as described herein have been provided by the present invention. Somewhat related devices and systems are disclosed in U.S. Pat. No. 3,238,448 to Wood et al.; U.S. Pat. No. 3,449,662 to Wood; U.S. Pat. No. 3,483,466 to Crouch et al.; U.S. Pat. No. 4,372,658 to O'Connor et al.; U.S. Pat. No. 4,447,777 to Sharp et al.; U.S. Pat. No. 4,598,250 to Lorenzi et al.; U.S. Pat. No. 4,649,343 to Birchak et al. and U.S. Pat. No. 4,675,604 to Moyer et al.